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Low number of X-DNA centimorgans shared between brother and I

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  • Low number of X-DNA centimorgans shared between brother and I

    I've just logged into my FTDNA account earlier today... and noticed that I only share 29 centimorgans with my brother on the X-chromosome. Which is pretty low value for a sibling AFAIK. I mean, is this unusual? If so, is there anyone else here who shares such a low X-DNA match with his/her full sibling(s)? Last but not least, what's the range (in terms of centimorgans) for X-DNA matches between full siblings/brothers?

  • #2
    With two full brothers, it is possible to inherit: no matching segments on the X chromosomes, some matching segments, or even a full match on the X. See "X Chromosome Recombination's Impact on DNA Genealogy," which points out a few things which I quote in the following list (my comments are in brackets in purple text):
    1. "The X chromosome men and women get from their mother is typically some combination of their mother's two X chromosomes." [so far, so good]
    2. "There are instances where a mother's X chromosomes do not recombine at all - she instead passes an exact copy of one of her X chromosomes to her child."
    3. "Brothers with the same mother may be full matches, partially full matches, or not matches at all (see below). This depends on how recombination did or did not occur from their mother." [the (see below) comment refers to an example on the page in the section "Half Matches vs Full Matches," under the statement in bold "A 100% match does not always indicate that X chromosome non-recombination has occurred. And neither does a 0% match.​"​]
    4. ​"Pay particular attention to male-male X-matches. Even small male-male full matches (perhaps even a few cM) can indicate a relatively close common ancestor." [this means X matches with males other than your brother]
    For #3, it's further explained elsewhere on the linked page that
    "Because full matches in men indicate portions of the X chromosome that have remained intact over potentially numerous recombination events, you should consider male-male X-matches with much more interest than any other matches.​"
    Bottom line: for two brothers with the same mother, they can match each other on the X the same way they match on the other 22 chromosomes - sometimes a small segment, sometimes a larger segment, and sometimes there is no match on a chromosome.

    edited to add: Here's an old blog post by Jim Owston (blog: The Lineal Arboretum)which shows how he matches on the X with his two brothers (and how they match each other) varies.The post is about phasing his segments, but just look at the amounts on the X chromosome that he shares with his brothers, under "My Specific X-Chromosome Issues."
    Last edited by KATM; 7 March 2023, 08:09 PM.